This invention relates to multilamp photoflash devices having circuit means for sequentially igniting the flashlamps and, more particularly, to an improved method for making a printed circuit board for a multilamp photoflash unit.
Numerous multilamp photoflash arrangements with various types of sequencing circuits have been described in the prior art, particularly, in the past few years. Series and parallel-connected lamp arrays have been shown which are sequentially fired by mechanical switching means, simple electrical circuits, switching circuits using the randomly varied resistance characteristics of the lamps, arc gap arrangements, complex digital electronic switching circuits, light-sensitive switching means and heat-sensitive switching devices which involve melting, fusing or chemical reaction in response to the radiant energy output of a proximate flashed lamp. The present invention is concerned with an improved method of making the sequencing circuitry for a relatively inexpensive photoflash unit of the disposable type. In particular, the present invention is particularly advantageous in photoflash arrays employing high voltage type lamps adapted to be ignited sequentially by successively applied high voltage firing pulses from a source such as a camera-shutter-actuated peizoelectric element.
A currently marketed photoflash unit of the last-mentioned type is described in U.S. Pat. No. 3,894,226 and referred to as a flip flash. The unit comprises a planar array of eight high voltage type flashlamps mounted on a printed circuit board with an array of respectively associated reflectors disposed therebetween. The lamps are arranged in two groups of four disposed on the upper and lower halves, respectively, of the rectangular shaped circuit board. A set of terminal contacts at the lower end of the unit is provided for activation of the upper group of lamps, while a set of terminal contacts at the top of the unit is operatively associated with the lower group of four lamps. The application of successive high voltage pulses (e.g., 500 to 4000 volts from, say, a piezoelectric source controlled by the shutter of a camera in which the array is inserted) to the terminal contacts at the lower end of the unit causes the four lamps at the upper half of the array to be sequentially-ignited. The array may then be turned end for end and again inserted into the camera in order to flash the remaining four lamps.
The flip flash circuit board comprises an insulating sheet of plastic having a pattern of conductive circuit traces, including the terminal contacts, on one side. The flashlamp leads are electrically connected to these circuit traces by means of eyelets secured to the circuit board and crimped to the lead wires. The circuitry on the board includes six printed, normally open, connect switches, that chemically change from a high to low resistance, so as to become electrically conducting, after exposure to the radiant heat energy from an ignited flashlamp operatively associated therewith. A thermal connect switch of this general type is described in U.S. Pat. No. 3,458,270 of Ganser et al. The purpose of these switches is to promote lamp sequencing and one-at-a-time flashing. The four lamps of each group are arranged in parallel, with three of the four lamps being connected in series with a respective thermal connect switch. Initially, only the first of the group of four lamps is connected directly to the voltage pulse source. When this first lamp flashes, it causes its associated thermal connect switch (which is series connected with the next, or second, lamp) to become permanently conductive. Because of this action, the second lamp of the group of four is connected to the pulse source. This sequence of events is repeated until all four lamps have been flashed. A major problem of this prior art circuit and other similar arrangements is that if one lamp short-circuits internally upon flashing, the successive parallel lamps of that group of four cannot be flashed; i.e., the remaining lamps are shorted out.
It has been found that this problem can be eliminated by the use of a radiant-energy-activated switch that is normally conducting and which becomes nonconducting subsequent to exposure to the actinic output of the flashlamp associated therewith. This disconnect switch is used in series with each of the lamps, except the last lamp, in a sequentially flashing parallel group of high voltage flashlamps. It may be used, if desired, in addition to the printed connect switches, which are normally open and which close upon actinic exposure. Hence, the modes of action of these two types of switches are opposite from one another -- the disconnect switch interrupts the igniting circuit of the lamp in series with it upon firing of that lamp, while the thermal connect switch establishes the igniting circuit for the next lamp upon being activated.
A number of radiant-energy-activated disconnect switches have been described in the prior art. Examples are U.S. Pat. Nos. 3,532,931 Cote et al., 3,726,631 De Graaf et al., 3,728,067 De Graaf et al., 3,692,995 Wagner, 3,666,394 Bok. A particularly fast acting disconnect switch is described in the aforementioned U.S. Pat. No. 4,017,728 of Audesse et al., wherein the switch element comprises a piece of electrically conductive, heat shrinkable, polymeric material which is positioned so as to be in operative relationship with the radiant output from the series connected lamp during the ignition thereof. Each switch element is attached at both ends to the lamp igniting circuit with the midportion of the element being spatially suspended to avoid contact with heat absorbing surfaces. Upon ignition of its associated lamp, the midportion of the switch element rapidly shrinks and separates in response to the radiant output of the lamp, thereby providing a quick-acting, reliable open circuit to high voltages.
According to one embodiment, these disconnect switches were employed in a photoflash array including a plurality of flashlamps mounted on a printed circuit board with a conductive reflector unit disposed between the lamps and circuitry. The circuit board is provided with a plurality of apertures respectively in alignment, via corresponding reflector apertures, with the flashlamps mounted thereon, and each length of conductive heat shrinkable material comprising a switch is attached to the circuit board so as to bridge a respective aperture therein. In this manner, the midportion of the material is spatially suspended to avoid contact with the circuit board. For silk screened circuitry it is advantageous to carry the conductive circuit pattern over the ends of the preapplied switch material, whereby the circuit material helps to anchor the switch ends to the circuit board substrate. During the production process, however, it has been observed that the circuit material covering the switch ends can be rubbed off or flaked off, causing a potential open circuit. Further, it was observed that the disconnect switch ends provided potential locations of shorting or arc-over to an adjacent reflector edge or nearby circuit traces.